Additive Manufacturing of Microfluidic Technologies via In Situ Direct Laser Writing

Abstract

Innovations in microfluidic technologies hold great promise for a wide range of chemical, biomedical, and soft robotic applications. Unfortunately, key drawbacks associated with soft lithography-based microfabrication processes hinder such progress. To address these challenges, we advance a novel submicron-scale additive manufacturing (AM) strategy, termed “in situ direct laser writing (isDLW)”. IsDLW is an approach that benefits from the architectural versatility and length scales inherent to two-photon polymerization (2PP), while simultaneously supporting the micro-to-macro interfaces required for its effective utilization in microfluidic applications. In this dissertation, we explore isDLW strategies that enable passive and active 3D microfluidic technologies capable of enhancing “on-chip” autonomy and sophistication. Initially, we use poly(dimethylsiloxane) (PDMS)-based isDLW to fabricate microfluidic diodes that enable unidirectional rectification of fluid flow. We introduce a novel cyclic olefin polymer (COP)-based isDLW strategy to address several limitations related to structural adhesion and compatibility of PDMS microchannels. We use this COP-based approach to print microfluidic transistors comprising flexible and free-floating components that enable both “normally open” (NO) and “normally closed” (NC) functionalities—i.e., source-to-drain fluid flow (QSD) through the transistor is either permitted (NC) or obstructed (NO) when a gate input (PG) is applied. As an exemplar, we employ COP-based isDLW to print an integrated microfluidic circuit (IMC) comprised of soft microgrippers downstream of NC microfluidic transistors with distinct PG thresholds. All of these microfluidic circuit elements are printed within microchannels ≤ 40 μm in height, representing the smallest such components (to our knowledge). Theoretical and experimental results illustrate on the operational efficacy of these components as well as characterize their performance at different input conditions, while IMC experimental results demonstrate sequential actuation of the microrobotic components to realize target gripper operations with a single PG input. Furthermore, to investigate the utility of this strategy for static microfluidic technologies, we fabricate: (i) interwoven bioinspired microvessels (inner diameters < 10 μm) capable of effective isolation of distinct microfluidic flow streams, and (ii) deterministic lateral displacement (DLD) microstructures that enable continuous sorting of submicron particles (860 nm). In combination, these results suggest that the developed AM strategies offer a promising pathway for advancing state-of-the-art microfluidic technologies for various biological and soft robotic applications.